Language is a hallmark of human beings. In the last 50 years, debates on language have given way to a new view of the process by which humans acquire language. One catalyst for theoretical change has been empirical studies on infants. In the last decade, researchers have not only charted when infants acquire knowledge about the properties of their native language, but how they do so, and this has caused a revision in linguistic and psychological theories. New research focuses on the phonetic units of speech, the consonants and vowels that form building blocks for words. Key advances from this laboratory are cross- language data showing that infants learn from exposure to language in the earliest periods of development and that this alters speech perception to assist language learning. Moreover, our studies show that early speech predicts later language, and that the clarity of mothers'infant-directed speech is linked to infants'speech perception abilities. Finally, brain measures on infants and adults listening to language suggest that, during early development, the infant brain "neurally commits" to the patterns of native language speech and that this both promotes future language learning as well as the decline in nonnative speech perception that occurs at the end of the first year of life. This work on early speech perception is impacting child development, neuroscience, neurobiology, and computational modeling. The early speech measures developed as a part of this project are being used in the study of developmental disabilities including autism, and may provide an early marker of the disability. The data prompted an extension of the Native Language Magnet model to incorporate neural commitment as the mechanism for developmental change. This theoretical position provides the background and framework for the studies in this proposal. Four converging lines of research are proposed to test the theory and further advance our knowledge of infant speech development: (a) speech perception development and its impact on language;(b) the brain correlates of early speech and language development, (c) the role of language input to children, and (d) brain plasticity and the "critical period" for language acquisition. The research will produce data that address theories of speech and language development and more general theories of the interface between biology and culture.